The invention concerns a double-wall fuel container of plastic material, for example as a motor vehicle fuel tank.
A double-wall fuel container such as a motor vehicle fuel tank of plastic material may typically comprise at least two elements in shell form, which are joined together by joining of the materials thereof.
In the context of fuel containers generally, it is known that single-layer fuel containers consisting of polyethylene are more or less permeable in relation to hydrocarbons, with the result that hydrocarbons diffuse through the wall of the tank and are discharged to the ambient atmosphere, which is undesirable. Fuel tanks are the subject of increasing demands in terms of the permeability thereof in relation to hydrocarbons.
Many attempts have been made to reduce the level of hydrocarbon emissions from fuel tank systems. To further those aims, a fuel tank may be made by a co-extrusion blow molding process, with the wall thereof comprising a laminate with one or more barrier layers which afford a low level of permeability for hydrocarbons. Such tanks generally satisfy the demands in relation to maximum admissible levels of hydrocarbon emissions.
However, problems still arise in this respect in regard to connecting openings in the tank, which are required to form a sealing seat for connection members, valves and the like, while in addition welded seams both in the case of fuel containers which are made up of a plurality of component parts made by an injection molding process and also in the case of fuel containers which were obtained by extrusion blow molding represent a possible source of emissions.
In an attempt to reduce the rate of permeation of a fuel container, for example DE 36 00 177 A1 proposes that the fuel container is made up in a double-wall structure comprising an inner container and an outer enclosure or casing which together form a sealed intermediate space with which there communicates a conduit for the discharge of gas or air. Admittedly, with such an arrangement, a large part of the hydrocarbons which diffuse directly through the tank wall can be trapped in a filter element, but here it is necessary for conduits to be passed through the outer casing a plurality of times, and the points at which the conduits pass through the outer casing in that way also represent sources of hydrocarbon emission. Such conduits and conduit connections which are disposed outside the container have to be optimised in terms of their emissions, and that is highly complicated and expensive and sometimes also encounters limits.
The endeavour therefore is to move as many connecting locations and conduits as possible into the tank or container. For that purpose for example DE 198 49 820 A1 discloses gas-tightly mounting in the region of the tank, on the outside thereof, at least one cover member which in conjunction with the tank wall delimits a space which is screened off with respect to the ambient atmosphere and within which connections, connecting conduits and the like are disposed. That cover member is gas-tightly welded to the wall of the tank. It is also possible to dispose therebeneath for example an activated carbon filter for receiving and separating off any free hydrocarbons which may occur.
A similar design configuration is to be found for example in WO 00/48859 where the tank is also gas-tightly welded to a large-area cover member in the form of a hood.
Endeavours are increasingly being made however to move all the connecting locations and conduits, if possible, into the interior of the container or tank. That is to be implemented particularly well in fuel containers which are made up of large component parts, for example two shell portions.
DE 198 53 097 for example discloses such a fuel container having a wall which is produced by an injection molding process from plastic material and which is of a two-part nature. Each half of the tank in turn comprises shell portions which are arranged one above the other and between which a slit or gap is kept free. Fuel vapors which pass into the gap are discharged therefrom and filtered. With such a tank, all installation components therewithin can be fitted under advantageous and convenient installation conditions, before the shell portions are welded together. Manufacture of such a tank however gives rise to the problem that the permeability properties of the plastic material used can scarcely be improved for example by adopting a multi-layer configuration for the tank wall. It is not possible for multi-layer shell portions of that nature to be produced by an injection molding procedure. Thermoshaping shell portions using a multi-layer semi-manufactured article usually gives rise to difficulties in terms of final manufacture as the processing temperatures for the individual components of the laminates which are usually employed are comparatively far apart.
An object of the present invention is to provide a fuel container of plastic material, which is of such a design configuration that it can be produced in particular inexpensively while as little fuel vapor as possible is permitted to pass into the ambient atmosphere through the container wall.
Another object of the present invention is to provide an improved fuel container of plastic material of a double-wall configuration, which affords improved resistance to permeability of hydrocarbon fuel vapors therethrough.
Yet another object of the present invention is to provide a plastic fuel container of double-wall structure which can be produced using a simple operating procedure.
In accordance with the principles of the present invention the foregoing and other objects are attained by a double-wall fuel container, for example as a motor vehicle fuel tank, of plastic material, composed of at least first and second elements in shell form, which are joined together by joining of the respective materials thereof. The elements in shell form are each in the form of a double-wall blow molding which are respectively co-extruded in a multi-layer configuration at least in a region-wise manner. A gap is kept free at least in a region-wise manner between the respective walls thereof.
A fuel container or tank produced in that way has various advantages over previous fuel containers, which are to be attributed more particularly to the fact that the individual constituent elements of the fuel container are co-extruded in a multi-layer configuration. On the one hand, that affords the possibility of reducing the permeability of the plastic material used, with measures and procedures which are known for that purpose. On the other hand, the arrangement according to the invention enjoys the advantages of multi-part containers, in terms of mounting and attaching conduits and connecting elements. All visible surfaces of the components can enjoy an outstanding surface nature, which is usually not otherwise the case in regard to the internal surfaces of fuel tanks which are conventionally produced by blow molding. In that situation the plastic material leaves behind otherwise relatively large flow markings, by virtue of the fact that no contact with a molding tool actually takes place in the interior of a blow-molded component. In addition, conventionally blow-molded fuel tanks suffer from irregularities in the wall thickness thereof, by virtue of different expansion distances on the part of different parts of the preform from which the blow-molded tank is produced. The fuel container according to the invention has thereover the advantage that the internal contour of the container has a defined smooth surface.
The elements in shell form may each of such a configuration that they form any division, including an asymmetrically extending one, in the assembled fuel container. For example an element in shell form can be in the form of a trough-like element or a bowl-like element, while the element which is to be fitted thereto to constitute the finished container can form a cover member.
In a preferred feature of the invention the double-wall elements are each in the form of a half-shell portion, with the shell portions being suitably welded together to form the container.
In a further preferred feature of the invention the walls of the elements forming the container can each be produced at least in a region-wise manner from a laminate with at least one barrier layer for hydrocarbons. Such barrier layers can be formed for example from EVOH (ethylene vinyl alcohol). By virtue of the double-wall nature of the elements forming the fuel container, it is possible to use less expensive barrier materials instead of EVOH, and that can contribute to further reducing the production costs of the fuel container. It is however also possible to provide a barrier layer in only one respective one of the two walls of the blow-molded elements. That could be easily implemented in the course of manufacture by the preform for producing the finished molded article being extruded only with a barrier layer extending over half the periphery thereof.
In a further preferred feature of the fuel container according to the invention, barrier layers for hydrocarbons are provided only in the wall regions of the elements, which form the outsides of the container.
A further preferred feature of the invention provides that there are means for venting the cavity of the elements which are in the form of hollow moldings. For example, appropriate openings for the intake and discharge of air can be provided at one or more suitable locations on the shell portions for making up the fuel container. In that case, it is desirable for the cavity of the elements to communicate with a filter element. In operation of a motor vehicle to which the fuel container according to the invention is fitted, it is possible for example to provide for active flushing of the cavity of the elements with air.
Passage configurations, subdivision means or partitions and other contours in the container wall can be afforded by welding wall regions of the elements, which are arranged in mutually superposed relationship, in other words, by the procedure known as wall-to-wall welding, whereby it is possible at the same time to provide for stabilisation of the fuel container. Passages at wall-to-wall welds of calibrated quality for passing conduits and connections therethrough are also an option in the design configuration according to the invention.
At least the regions, which form the inside wall of the container, of at least one element, preferably the element forming the top side of the container in the position of use thereof, can be of a profiled nature to receive and/or fix conduits and other fitments, and this can also be achieved by the above-mentioned wall-to-wall welding.
Alternatively, it is likewise easily possible to provide insert members for receiving conduits or fitments, such as for example of a reservoir, which involve an intimate join to the inside wall of the container in the blow molding procedure.
It can further be provided that mutually superposed wall regions are welded together, forming conduits and/or ducts therethrough, so that the installation of separate conduits becomes at least in part superfluous.
Finally, it is also possible to provide that the elements or shell portions are welded together in such a way that, in the region of the welded seam, they form between them at least one vent duct which extends at least in part around the periphery thereof and which can be vented in a similar manner to the above-mentioned cavity between the walls of the elements making up the fuel container. The permeability of the finished fuel container in relation to hydrocarbons can be still further reduced in that way.
The two shell portions of the fuel container can be joined together for example by heating element welding. Other ways of making the joins, for example by adhesive, can also be envisaged within the framework of the present invention.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of a preferred embodiment thereof.